They called it "bitch in a box". On a baking hot day last August, a black Mercedes sedan pulled up at the US army base in Ramadi and two US interrogators dragged an Iraqi man out of the boot. He was gasping for air.

"They kind of had to prop him up to carry him in. He looked like he had been there for a while," said a US soldier who witnessed the Iraqi's arrival in the custody of American interrogators wearing desert camouflage but no identifying insignia.

Such coercive interrogation techniques are widespread in Iraq. The Guardian has learned of ordinary soldiers who were instructed to use sleep deprivation on prisoners, and taught to perform mock executions.

The Guardian also spoke to soldiers who served at Abu Ghraib, in units not linked to the photographs of abuse which surfaced two weeks ago, who said prisoners were routinely humiliated by US troops venting their frustration.

Other troops witnessed procedures they believed to be wrong, but felt powerless to intervene.

The soldier who watched the gasping Iraqi emerge from the Mercedes said he saw a similar episode later in August. "That was the normal procedure for them when they wanted to soften up a prisoner: stuff them in the trunk for a while and drive them around," said the soldier, who asked not to be named. "The hoods I can understand, and to have them cuffed with the plastic things, that I could see. But the trunk episode, yes, I thought it was kind of unusual."

He added: "It was like a sweatbox, let's face it. In Iraq, in August, it's hitting 120 degrees, and you can imagine what it was like in a trunk of a black Mercedes."

Accounts such as these now beginning to emerge challenge the widespread presumption among Americans that the abuse was confined to a handful of troops at a single facility - the unfortunate outcome of an increasingly violent insurgency and a swamped prison system.

That hopeful notion - that the abuse was perpetrated by a few "bad apples" in the US military establishment - no longer holds. Critics of the Bush administration say the president and other officials sent a powerful message after the September 11 terror attacks that the previous standards of law no longer applied.

"The attitude that was communicated started from the highest levels and was sent on down the chain. It created an overall climate in which adversaries were dehumanised, the distinction between suspect and known perpetrator was effaced, and the overall message was that international law or domestic niceties get in the way of doing quote 'what we had to do', said Rosa Ehrenreich Brooks, formerly a senior adviser on human rights to the State Department. "When that is the message from the top it enables all sorts of bad behaviour."

The International Committee for the Red Cross has documented abuse at holding facilities across Iraq, not just Abu Ghraib. Cliff Kindy, a member of the Christian Peacekeepers Teams, which visit Iraqi detainees, said that until the horrific pictures emerged from Abu Ghraib two weeks ago, human rights workers believed that conditions at this facility were better than elsewhere. "The idea is once you got a permanent facility, people would be less likely to be treated improperly, and torture was less likely to happen," he said.

But the resort to coercive techniques by US personnel predates the war in Iraq, according to the New York Times yesterday, which reported that CIA interrogators used "waterboarding" - a euphemism for forcing a prisoner under water until he believes he will drown.

The technique was used on Khalid Sheikh Mohammed, who is believed to have helped plan the September 11 attacks. The pattern appears to have been transmitted through the opening weeks of the occupation of Iraq.

When the infantry units from the Florida National Guard arrived at the Assad airbase north-west of Baghdad in early May last year, they discovered they were to run a makeshift prison camp where detainees were confined in a vast aircraft hangar, with cells marked off with concertina wire.

Prisoners wore hoods made out of sacking used for sandbags. The Guard units were ordered to keep those suspected of being combatants awake for future interrogation, and instructed in techniques of sleep deprivation by three interrogators.

The interrogators were not in regular army uniform, and the soldiers never learned their real names.

"We had a sledgehammer that we would bang against the wall, and that would create an echo that sounds like an explosion that scared the hell out of them," said Camilo Mejia, a member of the Florida National Guard who has applied to the Pentagon for status as a conscientious objector.

"If that didn't work we would load a 9mm pistol, and pretend to be charging it near their head, and make them think we were going to shoot them. Once you did that, they did whatever you wanted them to do basically."

In a statement, written to support his conscientious objector application, Sgt Mejia writes that a platoon leader objected to their new duties, only to be told that his stand could end his military career.

"The way we treated these men was hard even for the soldiers, especially after realising that many of these 'combatants' were no more than shepherds," he writes.